The present invention relates to optical communication systems. More particularly, the present invention relates to an optical window signal generator device which produces an optical output signal in response to an activation and a deactivation optical pulse.
In many optical systems, it is useful to generate window pulses in response to controlling light pulses. These pulses can serve for example to activate the optical windows used for the sampling, interruption, and starting functions of digital optical signals. A window pulse, also referred to as an optical output signal, begins when the apparatus producing the window pulse receives an activation pulse. The window pulse terminates when the apparatus producing the pulse receives a deactivation pulse.
A window signal is essentially a step pulse of extended duration. It has three principal characteristics: the duration of the window, the rise time, and the fall time of the step generated. The rise time indicates how rapidly the window signal reaches its proper amplitude after the activation pulse is received, and the fall time indicates how rapidly the window pulse terminates after a deactivation pulse is received. For applications to optical communications systems and switching systems, it is important that the duration and rise time of the step signal generated be compatible with the bit rate of the transmission that is being sampled. In particular, if it is desired to sample a digital signal without interrupting a bit, the rise time must be 1 or preferably 2 orders of magnitude faster than the duration of the bit itself. The required duration of the window pulse depends on the length of the signal that must be sampled. For example, if it is desired to sample an entire cell of a packaged transmission, the duration of the window can be hundreds of times the duration of a single bit. Devices performing this sampling are useful in many digital subsystems, and are used in converters, counters and optical digital computers. It is very important in view of -these applications to control the rise time and duration of the window signal accurately.
Current optical switching systems cannot effectively meet these requirements and are unable to produce an optical window signal having a rise time, duration and fall time appropriate for the purposes listed above.
Another problem of the current methods is that there are no simple ways for controlling the generation of optical window signals, which must be activated and deactivated by simple pulses of light.
U.S. Pat. No. 5,537,243 to Fatehi et al. describes an all optical flip-flop achieved by employing two optical amplifiers arranged so that they together operate in only one of two stable states at a given time. In a first stable state of operation, the first optical amplifier behaves as a laser having a first characteristic wavelength. When an optical signal pulse is received at the input of the first optical amplifier the arrangement is switched to a second stable state of polarization in which the second optical amplifier behaves as a laser having a second characteristic wavelength, where the first and second characteristic wavelengths are at least nominally different. The arrangement is switched back to the first stable state when an optical signal pulse is received at the input of the second optical amplifier.
U.S. Pat. No. 5,007,061 to Odagawa discloses a bi-stable semiconductor laser diode device which has means for irradiating a reset light which stops the delivery of a lasing light from the laser. The laser includes an active layer comprising a gain region in which a stimulated emission occurs to obtain an optical gain, and a saturable absorption region in which no stimulated emission occurs so as not to obtain an optical gain at the lasing wavelength. The laser is reset by irradiating the gain region of the laser with a light having such a wavelength that it is amplified by stimulated emission.
Shimizu et al. in Optics Letters, Vol. 17, No. 18, Sep. 15, 1992, p. 1307-09, discuss a technique for the external frequency translation of light waves, which enables the stepwise sweeping of an optical frequency in time over a wide range. The frequency translator described is composed of an optical pulse modulator and an optical ring circuit containing an acousto-optic frequency shifter and an optical amplifier. The pulse launched into the ring circuit undergoes a frequency shift for each complete trip around the ring circuit, and the frequency is translated considerably from the original input pulse.
Shimizu et al. in Applied Optics, Vol. 32, No. 33, Nov. 20, 1993, p. 6718-26 and in Applied Optics, Vol. 33, No. 15, May 20, 1994, p. 3209-19 report a technique for the external frequency translation of light waves, permitting stepwise sweeping of an optical frequency over a wide range with high linearity with respect to time. The papers report experimental and theoretical analysis of the device.
U.S. Pat. No. 5,500,762 to Uchiyama et al. describes a light frequency control apparatus comprising a light pulse signal generating mechanism; a light signal generating mechanism for generating a light signal with a staircase-shaped frequency shift based on the number of cycles of a loop within which the light pulse signal propagates; and a mechanism for generating a dummy light when the signal light level becomes zero and for supplying the dummy light to a light amplifying mechanism. The purpose of this device is to increase the number of loop cycles of a light pulse signal within the light frequency shifter by improving the stability of and by shaping the output signal, and also to conduct a frequency shift over a wide range, as well as conversion of a stable light frequency.
U.S. Pa. No. 5,581,389 to Lee et al. describes a light frequency control apparatus comprising a light pulse signal generating mechanism; a light frequency shifter for circulating this light pulse signal a predetermined number of times, delaying the light pulse signal at each cycle thereby sequentially shifting and outputting the light pulse signal; an extracting mechanism for extracting the light pulse in the second half of a cycle, and a polarization control mechanism in the light frequency shifter for controlling the polarization of the circulating pulse based on the amount of attenuation of the light pulse signal outputted by the extracting mechanism. This device is intended to provide a light frequency control apparatus which can increase the number of cycles of a light signal by reducing the polarization dependency of the light frequency shifter to produce a stable light signal.
U.S. Pat. No. 4,738,503 to Desurvire et al. describes a fiber optic recirculating memory comprising a splice free length of optical fiber which forms a loop that is optically closed by means of a fiber optic coupler. The coupler couples an optical signal input pulse to the loop for circulation therein, and outputs a portion of the signal pulse on each circulation to provide a series of output pulses. A pump source is included to cause stimulated Raman scattering in the fiber loop and thereby cause amplification of the circulating signal pulse.
U.S. Pat. No. 5,533,154 to Smith describes an optical memory for storing optical signals of a first wavelength. FIG. 1 in Smith shows a known optical memory configuration. Its basic elements consist of a long loop (25 km in the example) of single mode fiber, a 3 dB fiber coupler and a single erbium-doped fiber amplifier (EDFA). The signal pulses to be stored are gated, via a first gate, into the loop via a port of the 3 dB coupler. A pulse train with a total duration of 120 ms just fills the 25 km long loop. As long as unity loop gain is maintained, the signal continues to circulate without decrease in pulse energy. A port of the 3 dB coupler acts as a tap for the memory, and a second gate selects the stored data after the desired time interval. FIG. 2(a) in Smith shows the decay of the output of the loop from the initial injection of the pulse train over 30 ms of delay to when the pump power to the loop EDFA is switched off. The limited resolution of the digital storage oscilloscope used does not reveal the individual 120 ms pulse trains. Applicants remark that by switching off the pump power of the EDFA in the loop a relatively slow decay of the pulse train power in the loop is achieved, due to the relatively slow decay time of the excited levels of Erbium in the EDFA.
Applicants have discovered that none of the conventional methods are capable of generating a satisfactory optical window signal, and in particular an optical window signal having the duration and rise time characteristics required to sample a digital optical signal, or for other applications requiring precise control of the duration, the rise time, and the fall time of the optical window signal generated.
Accordingly, the present invention is directed to an optical window signal generator device that substantially obviates one or more of the problems due to the limitations and disadvantages of the related art.
Additional features and advantages of the invention will be set forth in the description that follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and obtained by the process and by the apparatus particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described, the invention is an optical window signal generator including a first beam splitter having first and second inputs and one output, a second beam splitter having an input optically coupled to the output of the first beam splitter and having a first output and a second output optically coupled to the second input of the first splitter to provide an optical feedback loop, and an optical amplifier included in the feedback loop. In the optical window signal generator, an activation optical pulse communicated to the first input of the first beam splitter is split into a leading portion of an output optical signal and into an optical feedback signal, respectively, at the first and second outputs of the second beam splitter. The feedback optical signal is amplified by the optical amplifier to sustain the output optical signal beyond the termination of the activation optical pulse. An optical element is also included in the feedback loop for suppressing the feedback optical signal in response to receipt of a deactivation optical pulse, thereby terminating the optical output signal.
In another aspect, the invention includes a method for generating an optical window signal, started by an optical activation pulse and terminated by an optical deactivation pulse, having the steps of communicating the optical activation pulse to, an optical feedback loop, splitting the optical activation pulse into a leading portion of an output optical signal and into a feedback optical signal, amplifying the feedback optical signal to sustain the output optical signal beyond the termination of the optical activation pulse, and to maintain self-sustaining amplification. The method also has steps for communicating the optical deactivation pulse to the optical feedback loop, and terminating the self-sustaining amplification in response to the optical deactivation pulse, thus terminating the sustained output optical signal.
In another aspect, the invention includes an optical window signal generator having an optical coupler for inserting an optical activation pulse in a feedback loop and for extracting an optical output signal from the feedback loop, and an optical amplifier and an optical element included in the feedback loop. The optical activation pulse is split by the coupler into a leading portion and a feedback optical signal, which is amplified by the optical amplifier and transmitted to the optical coupler to sustain the optical activation pulse. The optical element suppresses the feedback signal in response to receipt of an optical deactivation pulse, so that the sustained optical output signal is terminated.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.